During the construction of wells, cement is used to secure and support casing inside the well and prevent fluid communication between the various underground fluid-containing layers or the production of unwanted fluids into the well.
Various approaches have been developed to prevent failure of the cement sheath. One approach is to design the cement sheath to take into account physical stresses that might be encountered during its lifetime. Such an approach is described in U.S. Pat. No. 6,296,057. Another approach is to include, in the cement composition, materials that improve the physical properties of the set cement. U.S. Pat. No. 6,458,198 describes the addition of amorphous metal fibers to the cement slurry to improve its strength and resistance to impact damage. EP 1129047 and WO 00/37387 describe the addition of flexible materials (rubber or polymers) to the cement to confer a degree of flexibility on the cement sheath. WO 01/70646 and PCT/EP03/01578 describe cement compositions that are formulated so as to be less sensitive to the effects of temperature on the cement when setting.
Nevertheless, the above-described approaches do not allow restoration of the zonal isolation, once the cement sheath has actually failed due to the formation of cracks or microannuli.
A number of self-healing concretes are known for use in the construction industry. These are described in U.S. Pat. No. 5,575,841, U.S. Pat. No. 5,660,624, U.S. Pat. No. 5,989,334, U.S. Pat. No. 6,261,360 and U.S. Pat. No. 6,527,849, and in the document entitled “Three designs for the internal release of sealants, adhesives, and waterproofing chemicals into concrete to reduce permeability”, Dry, C. M., Cement and Concrete Research 30 (2000) 1969-1977.
Nevertheless, none of these self-healing concretes are immediately applicable to well cementing operations because of the need for the material to be pumpable during placement.
“Self-healing” cement systems were finally developed for oil and gas industry applications. They are described in US 2007/0204765 A1 or WO 2004/101952 A1. These systems deal with the issues of deterioration of the cement sheath over time in a well.
Despite the many valuable contributions from the art, improved pumpable compositions that would be intended to be placed downhole, in a hydrocarbon reservoir, in a water reservoir, in a reservoir containing brines, in a carbon dioxide environment such as, for instance, a carbon dioxide environment encountered in a well in contact with a reservoir for the storage of carbon dioxide gas, thus allowing long-term isolation and integrity of wells in contact with CO2 would be desirable. Hence, such cracks or micro-annuli may create preferable pathways for hydrocarbon fluids or CO2 fluids, such as CO2 saturated brine, gas or supercritical CO2, wet or dry CO2, to migrate to surface.